Electrostatic coating apparatus



Oct. 24, 1950 M|LLER 2,526,763

ELECTROSTATIC COATING APPARATUS Filed'llay 20, 1946 4 Sheets-Sheet 1INVEN TOR. Elva/Pr P N/LI. ER

Oct. 24, 1950 a P. MILLER 2,526,763

ELECTROSTATIC COATING APPARATUS Filed lay 20, 1946 4 Sheets-Sheet 2 l2 La PT 4531-, L

IDLLLJ r I 8 INVENTOR.

5 I Enn'rPM/L H? Oct. 24, 1950 Filed May 20, 1946 E. P. MILLER 2,526,763

swcmosunc comma APPARATUS 4 Sheets-Sheet 4 INVENTOR EHER) P Mu. ER BYPatented Oct. 24, 1950 ELECTROSTATIC COATING APPARATUS- Emery P. Miller,Indianapolis, Ind., assignor, by

mesne assignments, to Ransburg Elcctro-Coating Corp., Indianapolis,Ind., a corporation of Indiana Application May 20, 1946, Serial No.671,037

6 Claims. 1

This invention relates to apparatus by which finely divided particles ofmatter are moved through space by electrostatic forces, and is generallyapplicable in any situation in which charged particles of mattersuspended in a fluid medium arecaused to move away from one and towardanother of two bodies between which a high difference of electricalpotential is maintained. For the sake of convenience, and not by way oilimitation, the invention is hereinillustrated and described as embodiedin apparatus by which charged particles of a coating material areelectrostatically deposited on an article to be coated and by which anarticle, after being coated with an excess of liquid coating material,is subjected to the action of an electrostatic field to remove theexcess.

Such apparatus is not broadly new. In utilizing it, the article issupported in spaced relation to an electrode, and an electrostatic fieldis created between the article and the electrode to promote the desiredmovement of coating-material particles. To create the high electricalpotential necessary for the establishment and maintenance of theelectrostatic field it has been customary to employ a transformer, theoutput of which is usually rectified in order that the accelerationimparted to charged particles in the field may be unidirectional. Inpractice, the spacing of the electrode and article, and thepotentialcifference maintained between them, depends on a number offactors, such as the desired uniformity of field distribution over thesurface of the article, the necessity of providing, in processes ofelectrostatic deposition, adequate space between the article andelectrode for the introduction and distribution of the finely dividedcoating material, reduction in the danger of sparking, etc.

The acceleration imparted to a charged particle of coating material inan electrostatic field varies in the same sense as thepotential-gradient of the field, and the potential gradient will in turnvary directly with the potential difference, and oppositely to thedistance, between the article and the electrode. For efllciency inpromoting the movement of coating-materials a high potential gradient isdesirable; but high potential gradients increase the possibilities of aspark between the electrode and article. In most instances, especiallywhere there is some possibility of accidental reduction in the distancebetween the electrode and article, it is customary to space the articlefrom the electrode by a distance about twice the spark-over distance atthe potential difference maintained. For example,

operating in air with a potential diflerence of 80,000 to 100,000 volts,the article would usually be spaced about eight to twelve inches fromthe electrode. Even with such a safety-factor, undesirable accidentalsparking may occur as the result of improperly supporting the article,swinging of the article when suspended from an overhead support, or fromsome other cause. In addition to danger of sparking between the articleand electrode, there is also the danger of sparking between the articleor the electrode and any object of different potential.

It is an object of this invention to reduce the danger of sparking in anelectrostatic field employed to produce movement of electricallycharged, macroscopic particles.

Another object of this invention is to provide an apparatus which willfunction to promote the movement of particles of material byelectrostatic forces and to reduce automatically the potentialdifference maintaining the field whenever the current exceeds apredetermined value. An additional object of the invention is to providean electrostatic particle-moving apparatus in which the averagepotential-gradient of the electrostatic field will remain approximatelyconstant in spite of variations, within limits, in the distance betweenthe terminals of such field. Still another object of the invention is toproduce apparatus oi the kind indicated which will produce the highpotential difference necessary to the maintenance of an effectiveelectrostatic field without storing large amounts of energy which mightcause serious consequences if quickly liberated. A further object of theinvention is to provide an apparatus having the above characteristicswhich will be simple in construction and durable in use and which can bereadily inspected, serviced, and repaired in the field.

In carrying out the invention, as applied to the coating art, thearticles to be processed are supported in spaced relation to anelectrode. In most instances, the articles are conveyed successivelypast the electrode by some suitable form of continuously movingconveyor. If the articlesare to be electrostatically coated, one or morespray guns are provided to discharge finely divided particles of coatingmaterial into the region between the electrode and article. If thearticles are to be electrostatically deteared, they are coated with anexcess of coating material before being brought into association withthe electrode. In both cases an electrostatic field is maintainedbetween the articles and the electrodes, such field acting in the firstcase to cause electrostatic precipitation of coating material on to thearticle and in the second case to remove excess coating material fromthe article.

In the preferred form of apparatus employed to create the potentialdiflerence necessary to maintain an electrostatic field between anarticle and an electrode, there are employed a group of rectifierswhich, separated by resistors, are connected in series. The freeterminal of the first of such rectifiers is connected to one terminal ofa source of voltage pulses and the other terminal of such pulse-sourceis connected directly to one of the elements between which theelectrostatic field is to be maintained. The other of such elements isconnected to the free terminal of the last rectifier. The rectifiers aresimilarly arranged in the series, and the anodes of adjacent rectifiersare interconnected by condensers. In addition, the cathodes of adjacentrectifiers are interconnected by condensers, and an additional condenseris connected between the first resistor and the second named terminal ofthe pulse source. The system of rectifiers, resistors, and condensersdescribed constitutes a voltage-multiplier in which each condenser, inthe absence of any loss or drain, would become charged to the voltage ofthe voltage pulses, with the result that the voltage between the articleand electrode would be the pulse-voltage multiplied by the number ofrectifiers.

The accompanying drawings illustrate the invention:

Fig. 1 illustrates isometrically an electrostatic coating apparatus anddiagrammatically a highvoltage source employed to create theelectrostatic field; Fig. 2 is a diagrammatic illustration ofelectrostatic detearing apparatus; Figs. 3, 4, and 5 are diagrammaticviews illustrating additional uses of the invention; Figs. 6 and 7 arediagrammatic views showing elements of the highvoltage source; Fig. 8 isa diagrammatic illustration of a modified apparatus for heating thefilaments of the rectifiers; Fig. 9 is a diagrammatic view of apparatusby which the range of control can be enlarged; and Fig. 10 is a viewillustrating a further application of the invention.

In Fig. l of the drawings I have illustrated electrostatic coatingapparatus comprising a conveyor l0 adapted to convey articles i I to becosted between two spaced interconnected electrodes I! each of whichpreferably comprises an open frame I3 and a plurality of fine wires I3stretched across such frame. One or more spray guns l4 discharge finelydivided coating material into the region between the two electrodes.

For the purpose of creating the necessary electrostatic field betweenthe article I I and the electrodes, a high potential difference ismaintained between the electrodes and the conveyor II, and the articlesare electricall connected with the conveyor. The finely dividedparticles of coating material discharged from the gun I acquire anelectrical charge and are caused, by the influence of the electrostaticfield, to move toward and precipitate upon the article II as it movesthrough the coating zone. In most instances, it is preferable to groundthe conveyor II and connect the electrode to one terminal of ahighvoltage source the other terminal of which is grounded. The use ofelectrodes embodying fine wires i3 is desirable, as the corona dischargefrom such wires is very elective in charging the particles of coatingmaterial. Suitable means may also be provided for rotating the articlesto be coated which preferably comprise a shoe or friction bar It androllers 16.

Fig. 2 illustrates diagrammatically another apparatus in whichelectrostatic forces are utilized to produce movement of particles ofcoating material. Here, a conveyor conveys over an electrode 21 articles22 which have previously been coated, as by dipping in a dip-tank 23,with an excess of coating material. By maintaining a high electricalpotential-difference between the article 21 and the electrode, excesscoating material on the article is caused to leave the article and movetoward the electrode. Here again, it is contemplated that the conveyorwill be grounded and the electrode connected to one terminal of ahigh-voltage source the other terminal of which is grounded.

The high-voltage source which, in combination with an article supportand an electrode, constitutes one feature of this invention, includes avoltage-multiplier, indicated at A in Fig. 1.

Such multiplier comprises a series of rectifiers 80, considered asherein described to be of the vacuum-tube type, equal in number to thevoltage-multiplication desired. The anode of each rectifier I. isconnected to the anode of the next rectifier of the series throughacondenser ll, while the rectifier-cathodes are similarly interconnectedthrough condensers 32. In the specific arrangement shown in Fig. 1,where it is assumed that the electrode i2 is to be positively charged,the cathode of each rectifier except the last of the series is connectedto the anode of the next succeeding rectifier through a resistor 33.

The anode of the first rectifier 3|! of the series is connected througha conductor 34 to one terminal 35 of a pulse-transformer 36 the otherterminal I! of which is connected through a condenser with the cathodeof the first rectifier. The transformer is provided with an intermediatetap I! so that it may serve as an autotransformer to multiply theamplitude of voltagepulses supplied over conductors ll and 42 connectedrespectively to the tap 39 and the transformer-terminal 31. A rectifier41, having its cathode connected to the transformer-terminal II and itsanode connected to the terminal I'l, serves to suppress or by-passone-half of each of the alternating-voltage waves created by thetransformer 38 leaving the other half-waves to be passed to the anode ofthe first rectifier 3| in the form of positive pulses 4|.

The high voltage generated by the circuit of Fig. 1 exists between thetransformer-terminal I1 and the cathode of the last rectifier 30. Inmost cases, it is preferable to ground the terminal l1 and the conveyorIll and connect the cathode of the last rectifier to the electrode I2,as by a conductor 46.

For convenience in the following description of operation, one set ofcorresponding terminals of the successive rectifiers 30 are designatedin the drawing by the letters a, b, c, etc. and the other set by theletters 11. q, r, etc. The individual condensers 3| and I2 and theindividual resistors are distinguished by subscripts designating therectifier terminals between which they are connected.

The high-voltage multiplier shown in Fig. 1, operates in the followingmanner: Voltage-pulses transmitted to the transformer 36 over theconductors H and I! are amplified by pulse transformer 36 to causepositive voltage-pulses N to be impressed across the first rectifier IIand the condenser ll. The frequency of the pulses l4 will equal that ofthe pulses supplied over the conductors 4| and 42, while their amplitudewill equal that of the latter pulses multiplied by the step-up ratiowhich the transformer 33 affords. Assuming for the present that none ofthe condensers is charged and that there is no drain from thehigh-voltage generator, for example, through the electrostatic field,the first voltage pulse 44 will cause current to now through the firstrectifier 30 to charge the condenser 33. This current will continue toflow until either the condenser 38 is charged to a voltage equal to thatof the pulse 44 or until the termination of such pulse. During the deadportion of the cycle, the rectifier 30 prevents the condenser 33 fromdischarging over the same path by which it was charged. However, as thecondenser 33 is connected in a series circuit with the resistor 33, thecondenser 3km, and the transformer 33, a portion of the charge initiallyimparted to the condenser 38 will be displaced during the dead part ofthe cycle to the condenser 3h. If the dead part of the cycle is ofsufficient duration and if the condensers 38 and 3| are of equalcapacity, the charge initially imparted to the condenser 38 will becomedivided equally between it and the condenser 3|ab, and the points p andb will both be at the same potential relative to ground. Whether or notthe dead part of the cycle and the constants of the circuit are such asto permit the points b and p to attain the same potential, the condenser3| will acquire a charge during the dead part of the cycle and apotential-difference will exist across it between the points a and bwhen the next voltagepulse 44 occurs.

When the second voltage-pulse 44 occurs, the potential at points a and pwill be raised to that of the pulse and the charge on the condenser 33will be replenished. However, the potential I difference across thecondenser slab will be added to the potential at point a, so that apotentialdiiference will exist across the resistor 33m, and current willfiow through the rectifier 30a; to the condenser 32 As a result, thecondenser 32 will acquire a charge. .During the following dead part ofthe cycle, a part of the charge on condenser 33 will be transferred tocondenser 3| as before, and a portion of the charge imparted to thecondenser 32 will be transferred to the condenser 3|bc over the circuitq-cb-p. In this manner, as the cycle is repeated, a charge on thecondenser 33 and on each of the condensers 32 is built up during theexistence of the voltage pulse and shared with a condenser 3| during thedead part of the cycle. This condenser-charging process continues untilthe terminal condenser 32, which has no associated condenser 3| withwhich to share its charge, becomes charged to a potential. equal to theamplitude of the pulses 44. When this occurs, the condensers 3|, 32, and33 will be fully charged and the potential difference across each onewill equal the amp itude of the voltage pulses 4|. Theoretically, thepotential difference between the conductor 46 and ground will equal theamplitude of the voltage pulse 44 multiplied by the number of rectifiers30; but because of inevitable losses, the voltage actually obtained fromthe system will never attain this theoretical value.

The rise in potential which occurs during the pulse at the points b, c,d, etc., creates across the resistors 33 potential differencesrepresenting energy available to increase the charges on the condensers3| and 32. During the transient charging process above described, thisavailable energy is employed to build the condensercharges up to normal;but after each condenser has become fully charged, such available energyis employed to replenish the charge lost by drain from the point 0. Asthe pulses are of fixed frequency and width, the energy available toreplenish the charges on the condensers is limited. Under any constantload, the multiplier will maintain at the point 0 a potentialcorresponding to the equilibrium condition existing when the chargesupplied on pulse equals the charge withdrawn by the load. An increasein load current, therefore, will immediately cause a lowering of theoutput voltage; while if the load current decreases to some fixed valuethe output voltage will gradually attain a new equilibrium value.

If the high potential at point i; is imposed through the conductor 46 onthe electrodes l2, an electrostatic field will be created between suchelectrodes and the grounded article I supported between them. Themagnitude of the current traversing this field will depend upon thepotential difference between the electrodes and the article and upontheir spacing. So long as the energy represented by the currenttraversing the electrostatic field equals the energy supplied to thesystem as a result of the voltage pulses H, the potential of theelectrodes l2 will remain substantially constant. However, if for anyreason the distance between the articles and the electrode shoulddecrease, the current traversing the electrostatic field would increase.The increase in field current could result only from a decrease in thecharges on the condensers 3|, 32, and 38, and would inevitably beaccompanied by a decrease in the potential of the electrodes l2. Byproper selection of the constants of the circuit, the field current canbe kept near but always less than the value which must exist before aspark between the electrodes, or any other charged body, and thearticle, or any other grounded object, can occur. If a direct connectioncould be instantaneously established between the point 0 and thetransformer-terminal 31, the maximum energy represented by the resultantcurrent would be no more than that stored in the condensers. If thepoint 0 and the terminal 31 should be permanently interconnected, themaximum instantaneous current which could flow through the connectionwould equal the amplitude of the pulses 44 divided by the sum of theresistances of the resistors 33.

Performance characteristics of the voltage supply apparatus will dependupon the resistance of the resistors 33, upon the capacity of thecondensers 3|, 32, and 33, and upon the amplitude, duration, frequencyand form of the pulses 44. There are set forth hereinafter the constantsof an apparatus which has been found suitable in producing a specificpotential difference between the point '0 and the terminal 31 of thetransformer 36. The following discussion is intended to set forth in ageneral way the considerations which will influence the design ofapparatus to meet any particular situation.

The resistance of the resistors, by controlling the current which flowsduring the dead part of the cycle, controls the rate at which electricalenergy is passed from the condensers 33 and 32 asasnes 7 avoided; as anycurrent fiowing through them during each pulse reduces the quantity ofelectrical energy transferred from the condensers 3| to the condensers32. For example, the relatively high potential which exists at point bdurfiers during each pulse and through the highresistance resistorsduring the dead part of each cycle. Accordingly the duration of eachpulse should be short relative to the length of the remainder of thecycle. Each pulse, however, desirably endures for a length of time greatenough to charge the condenser 38 to pulse-voltage.

Since the potential at the point 0 represents the sum of the voltagesacross all the condensers l2 and 38, and since all such condensersreceive increments of charge only during the voltage pulses 44, anydrain from the point 1) will cause its potential to drop graduallyduring the dead portion of each cycle between successive voltage pulses.Obviously, the condensers employed in the system should possess adequatecapacity to prevent the voltage at point 1) from dropping to anobjectionable extent during the dead portion of each cycle. It may benoted in this connection that inductance which will inevitably bepresent in the system will tend to reduce intracyclepotential-variations at the point v.

As the quantity of energy which must be stored in the condensers toprevent an undesirable potential-drop at the point 1) during the deadportion of the cycle will depend upon the duration of each such deadportion, the necessary capacities of the condensers can be reduced byincreasing the frequency of the pulses N. As previously noted,condensers of small capacity are desirable in order to decrease theenergy stored in the system, and hence it is necessary to use relativelyhigh pulse-frequencies if stored energy is to be reduced to a safeminimum.

It is desirable that the voltage pulses ll be as nearly square in formas is practicable. To make apparent the reason for such a square form,let it be recalled that at the termination of each pulse the potentialat the point a drops to zero and remains at that value (except for thevoltage drop across the transformer 38) during the dead part of thecycle. During the next voltage pulse, the voltage across the resistor33b which causes charging of the condenser 32, rises with the potentialat point a. The less rapid the rise of potential at point a, the smallerthe increment of charge which is transferred from the condenser ii, tothe condenser 32 It may be noted further that after the potential atpoint a has attained that previously existing at point p furtherincrease in the potential at point a is transmitted to the point pthrough the rectifier SI and hence does not result in any increase ofthe voltage across the resistor 33. Therefore, the more rapidly thepotential at the point a rises at the beginning of each voltage-pulse44, the greater will be the increment of charge which is transferredfrom the condenser list to the condenser 32 during the pulse. It is alsodesirable that the termination of the pulse be as abrupt as possible;since the transfer of charge from the condenser 8atothecondenserllawouldbeoppoeedby any potential generated in thetransformer 88.

While it was assumed in the above description of operation that thevoltage-pulses 44 were positiveinsign,itistobeundersiood that thesysteinwill operate in the same way to produce a potential difference betweenthe point 0 and the transformer-terminal 81 if the voltage pulses N arenegative in sign. In fact, in coating processes it is preferred that thevoltage pulses be negative; as the precipitation of coating material onthe article seems to be somewhat more effective when the electrode-wiresI! are charged negatively relative to the article than when theyare'charged positively relative to the article. As it is generally moreconvenient to ground the conveyor II, and through it the article ll,than to ground the electrode II, the preferred negative charge on theelectrode wires is most conveniently secured by employing voltage-pulses44 which are negative in sign. In the preferred form of the apparatusillustrated in Figs. 6 and "l, the pulses 44 are negative in sign.

That preferred form of apparatus, which is shown in its entirety inFigs. 6 and 7, comprises a saw-tooth wave generator D, an amplifier andpulse-limiter E, and a power pulse generator 1'. The saw-tooth wavegenerator D comprises, as will be evident from the drawing, a blockingoscillator capable of producing waves of the form indicated at It in itsoutput circuit when there is no drain thereon. Such generators are wellknown, and consequently need not be described in detail here.

The amplifier and pulse limiter E comprises a vacuum tube ll, here shownas a tetrode, the control grid of which is coupled to the output circuitof the saw-tooth wave generator D. The power supply to the plate circuitof the tube 85 includes an inductance 5i, and the effective grid-bias ofsuch tube will be determined by the value of the resistance 50. Byappropriate selection of constants the control grid of the tube I! willbe given a negative bias such that when the negative signal from thegenerator D is added to it the tube will swing beyond cut-off. As aresult, the amplitude of variations in plate potential will becontrolled by the grid-bias and will to a large extent be independent ofthe amplitude of the input pulses imposed on the control grid.

Because of the drain from the output circuit of the saw-tooth wavegenerator D, when such generator is connected to the amplifier andlimiter E, the saw-tooth wave 59, which otherwise would exist in theoutput circuit of the generator D, is distorted toward the more or lessidealized pulseform indicated at I in Fig. 6. Such negative pulsesimposed on the control grid of the tube ll cause positive, andamplified, pulses CI in the output circuit of such .tube. The frequencyof the pulses 6| will of course correspond with the lirequency of thewave generated by the oscillator of the generator D. The amplitude ofthe P111 1 which is determined by amplification factors, constants ofthe generator D and amplifier-limiter E, and the B-voltages supplied tothose two elements, will ordinarily be several hundred volts.Thewidthofthepulsesil willdependuponthe value of the plate inductancell, increasing as the value of such plate inductance increases.

The power pulse generator 1" embodies a vacuum tube 8', shown as atetrode, upon the grid of which the positive pulses Ii from theamplifierlimiter E are imposed. Thetube H has a fixed negative biassumcien lr 1 8 to render it nonconducting during the intervals betweensucces sive positive pulses 8i.

During these non-conducting intervals the condenser 61 is charged bybeing connected to the "B" voltage supply through resistor 62, choke 63and pulse transformer primary II39. When the triggering pulse 6| isapplied to the grid of tube 85 its grid is driven highly positive forthe duration of the pulse. The tube is thus highly conducting duringthis period and represents a low impedance path to ground for the chargestored on condenser 61. A surge of current thus fiows through thecircuit 3|-l04I--65 and ground. In this way a pulse of voltage issupplied to the primary of the pulse transformer which is equal to theplate supply voltage minus the voltage drop across tube 65.

It is these voltage pulses 86 which are imposed over conductor 4|between the intermediate tap II and the grounded terminal 31 of thepulsing transformer 36 previously described.

As in the case of the voltage-supply system illustrated in Fig. 1, thevoltage pulses 44 generated in the pulse transformer 36 are transmittedto the voltage multiplier over the conductor 34. Since, in the case ofthe apparatus illustrated in Figs. 6 and '7, it is desired that the highpotential produced be negative relative to ground, the rectifier 43associated with the transformer 36 is reversed "from the position shownin Fig. 1 so that it will permit only negative pulses to pass over theconductor 34. As the pulses supplied to the multiplier shown in Fig. 7are negative, the rectiflers 30 are likewise shown as reversed from thepositions shown in Fig. l.

The rectifiers 30 shown in Fig. 7 are desirably of the vacuum-tube type.In supplying them with filament-heating current. it is necessary to doso by some means which will not interfere with the existence of highpotential differences between successive stages of the multiplier. Themeans for heating the rectifier-filaments of the tubes 30 comprises whatis in effect a high-pass filter network one leg of which includes theserially connected condensers 3| and the other leg of which includes acorresponding number of serially connected condensers 10. The filamentof each rectifier 30 is connected, in series with an inductance llacross the two legs of the filter network, one filament and itsassociated inductance being connected in each stage of the filter toconstitute a transverse leg of the filter network. The filter networkterminates at one end in a terminal impedance 12 and is connected at itsother end to a load coil I3 inductively coupled to the plate circuit ofan oscillator 14, shown as of the conventional tuned plate, magneticfeed-back type.

By way of illustration and not limitation, the constants of a suitableapparatus are as follows: The pulse generating system D--EF is capableof producing voltage pulses 86 having a frequency of 10,000 cycles persecond and an amplitude of 3,000 volts, and the pulse transformer 36 hasa step-u ratio of -to-1, with the result that the amplitude of thepulses ll supplied to the voltage-multiplier will be 15,000 volts. Theduration of each pulse, 68 is so controlled that each pulse 44 willendure for about 5% of each complete cycle. In that high-voltage source,each of the condensers 3|, 32, and 38 has a capacity of 0.0005 mfd.,while the resistors 33 have a resistance of 1 megohm. In thefilamentsupply circuit, each of the condensers has a capacity of 0.0005mid, each of the inductances II has a value of 100 microhenries, and theterminal impedance 1! has a value of 600 ohms. The oscillator 14 has anoutput in the neighborhood of 25 watts at a frequency of 1.5 mc.Employing seven condensers 38 and 32, as shown in the drawings, such ahigh-voltage source is capable of maintaining a potential difference inthe neighborhood of 100,000 volts between the electrode and the articlesbeing treated under the influence of the electrostatic field.

In addition to its general applicability in other fields, a high-voltagesource of the type herein illustrated and described has particularadvantages in electrostatic coating and detearing apparatus, because ofits inherent ability to reduce the potential difference between theelectrode and the article being treated when the distance between suchelectrode and article decreases or when, for any other reason, anincrease in field current occurs. Thus, where articles of non-circularcross-section, such as the articles 10 shown in Fig. 3, are rotated asthey pass between discharged electrodes i2 by a mechanism such as thatillustrated in Fig. 1, the distance between each electrode and theadjacent surface portion of the article will vary. If a constantpotential difference were maintained between the article and electrode,variations in such distance would cause variations in the averagepotential gradient of the field. However, by maintaining the fieldthrough the use of the apparatus above described, the potentialdifference between the articles and electrode will vary in the samesense as the distance between them and variations in the averagepotential gradient of the field will be materially reduced in extent.Similarly, where, as in Fig. 4, articles 11 and 18 of different sizesare to be passed between the electrodes, variations in average potentialgradient can be materially reduced by the use of the apparatus of Fig.7. In each of these cases, the electrodes i2 may be set to provide afield of satisfactory strength over the surfaces of the articles whenthe distance between the article-surface and the electrode is a maximum.When such distance decreases, as by reason of rotation of the article 16(Fig. 3) or by reason of the entry of alarger article into the field(Fig. 4), the resultant tendency of the field current to increase willcause a decrease in the potential difference responsible for the field.Thus it is possible to maintain satisfactory field-strength over thearticle-surface at all times while greatly reducing the possibility ofsparking.

Another apparatus in which the high-voltage source above described maybe used with advantage is shown in Fig. 5. Here, each individual article19 being treated in the field has varying dimensions transversely of theline of travel, and two independent electrode systems and 8 i eachhaving its own high voltage source A, are employed. The voltages and thespacing of the electrodes from the path of article travel areestablished to provide a field of satisfactory strength over thesmaller-dimensioned portion of each article. When suchsmaller-dimensioned portion is opposite either electrode, the voltage ofthat electrode will have its normal value; but when the largerdimensioned portion of an article is opposite either electrode, thevoltage of such electrode will automatically drop. Thus, it is possibleto maintain a reasonably uniform field strength over a surface whosedistance from elec trodes may vary widely.

Still another form of electrostatic coating apparatus in which thevoltage-multiplier above assures l ldescribedcanbeusedtoadvantageisthatillmtrated in Fig. 10. In thatfigure, an article OI is to be coated with coating material dischargedfrom a manually manipulated spray gun It with which a dischargeelectrode 81 is movable. The high voltage source including themultiplier is connected between the article I and the electrode 81 sothat the coating-material particles discharged from the gun will enteran electrostatic field, acquire a charge, and be urged toward thearticle for precipitation thereon. In this case, the electrode '1 ratherthan the article II is shown as grounded. The voltage multiplier, whenused as a means to create the electrostatic fieldbetween the electrodeand article, serves to maintain a field of reasonably uniform potentialgradient and to reduce greatly the danger of sparking and of injury tothe operator.

Other advantages of the high-voltage source described herein result fromthe fact that different potentials can be obtained by tappin thehigh-voltage source at any of the points p, q, 1',

etc., and the fact that such a source could be approached by anygrounded obiect andas approach took place a corresponding reduction involtage would occur, so that serious shock or discharge would never bepossible.

In the filament-heating arrangement shown in Fig. 8, the filaments ofthe rectifiers II are connected in a series circuit including thecondensers 3|, the load coil 13 (Fig. '7), and preferably additionalcapacity which, for convenience in manufacture and maintenance, may be abank of condensers 15 similar to the condensers ii. The series-circuitjust referred to is adapted to be tuned to the frequency of theoscillator 14, as by means of a variable condenser it connected inparallel with the coil II. By varying the adjustment of the condenserII, the current supplied to the rectifier-filaments may be varied asdesired. To compensate for radiation losses which might interfere withuniform heating of the filaments, resistors 11 may be connected in shuntwith. the filaments of those rectifiers nearest the coil 13.

To augment the range of automatic control of potential produced, theapparatus previously described may be arranged so as to cause areduction in the amplitude of the pulses 44 upon an increase in fieldcurrent. One arrangement suitable for that purpose is illustrated inPig. 9. As there shown, a fixed resistor 9| and a v'ariable resistors!connectedinserieswitheachotherare inserted in parallel with thecondenser 38, and the intermediate terminal of such resistors isconnected to the positive terminal of the source 9: which provides thenegative bias for the grid of tube I. In such an arrangement, anincrease in current traversing the electrostatic field, by increasingthe voltage-drop across the resistor",willcauseanincreaseinthenegativebias imposed on the grid oi tube II,thus reducing the amplitude of the pulses l and I4. Since the maximumpossible voltage across the voltage multiplier is equal to the number ofcondensers l2 and is multiplied by the amplitude of the pulses ll,reduction in such pulse-amplitude will reduce the potential difi'crencebetween the electrode and ground.

The invention claimed is:

1. In apparatus for coating articles, a conveyor for moving articlu overa predetermined path, an electrode mounted in spaced relation to saidpath, a source of unidirectional voltage pulses and a multiplying meamassociated with said 12 source and including a plurality of capacitorsconnected in series and having end terminals which are electricallyconnected respectively to sad electrode'and an article to be coated toestablish an electric field therebetwcen, and the intermediate terminalsof which are electrically associated with said source whereby thevoltage difference maintained between said electrodeandanarticletobecoatcdisequaitothesumof the voltage difi'erences acrosseach of the capacitors of the series and the electrical charge on eachcapacitor of the series is adapted to be changed in inverse proportionto the field current flowing between said electrode and an article to becoated.

2. In apparatus for coating articles, a conveyor for moving articlesover a predetermined path, an electrode mounted in spaced relation tosaid path, a source of unidirectional voltage pulses, an outlet terminalelectrically connected to said source for maintaining an electrostaticfield between said electrode and an article in the predetermined path,means for electrically connecting the article being coated to oneterminal of said source a mmtip yins circuit intermediate the otherterminal of said source and said outlet terminal comprising a pluralityof stages each including a capacitor. a resistor and a vacuum tuberectifier connected in delta, each stage being connected to two adjacentstages in such a way that its resistor is common to one of the adjacentdeltas and its rectifier common to the other adiacent delta, said pulsesource replacing the resistor of the initial delta and said outletterminal being the common connection between the rectifier and thecapacitor of the final delta.

3. In apparatus for controlling the movement of macroscopic particles ina gaseous medium, at least two electrodes spaced in the gaseous medium,a source of unidirectional voltage pulses, a multiplying meansassociated intermediate said source and said electrodes including aplurality of capacitors connected in series and having end terminalswhich are electrically connected re- I spectively to said electrodes toestablish an elecl. the sum of the voltage differences across each ofthe capacitors of the series and the electrical charge on each capacitorof the series is adapted to be changed inversely to the field currentflowing between said electrodes.

4. In apparatus for controlling the movement of macroscopic particles,at least two electrodu separated in a gaseous medium, means forintroducing particles into the medium between said electrodes, a sourceof unidirectional voltage pulses, .a transformer for said pulses, afirst rectifier connected between the outlet terminals of saidtransformer, a series of capacitors one end of which is directlyconnected to one terminal of said first rectifier and the other end ofwhich isconnectedthrougharectifiertoailrstofsaid electrodes, each ofsaid capacitors being shortedbyarectifierandaresistanceinscrles,saidrectifierofsaidseriesbeingconncctedtothetermimlof said capacitor closest to said source, a

second series of capacitors one end or which is connected directly'tothe other terminal of said first rectifier and the other end of which isdirectly connected to said first electrode, the intermediate connectionsof said second series being lntur'nconnectedtotheterminalconimonto boththe rectifier and resistance which shorts the capacitors of said firstseries.

5. In apparatus for controlling the movement of macroscopic particles,at least two electrodes separated by a gaseous medium, means forintroducing particles into the medium between said electrodes, a sourceof unidirectional voltage pulses, an outlet terminal electricallyconnected to one of said electrodes, means for electrically connectingthe other of said electrodes to one terminal of said source, a.multiplying circuit intermediate the other terminal of said source andsaid outlet terminal comprising stages each including a capacitor, a.resistor and a rectifier with a filament connected in delta, each stagebeing connected to two adjacent stages in such a way that its resistoris common to one of the adjacent deltas and its rectifier common to theother adjacent delta, said pulse source replacing the resistor of theinitial delta and said outlet terminal being the common connectionbetween the rectifier and the capacitor of the final delta, and highfrequency means for heating filaments of said rectifiers.

6. In apparatus for controlling the movement of macroscopic particles ina gaseous medium, at least two electrodes spaced in the gaseous medium,a source of unidirectional voltage pulses includ- 14 ing a vacuum tubewith a control grid, a multiplying means associated intermediate saidsource and said electrodes and including a series of ca pacitors the endterminals of which are electrically connected to said electrodes toestablish an electric field therebetween and the intermediate terminalsof which are electrically associated with said source, and resistancemeans connected across one of said capacitors and to said control gridfor controlling the output of said vacuum tube inversely to the fieldcurrent flowing between said electrodes.

EMERY P. MILLER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,855,869 Pugh Apr. 26, 19322,042,181 Knowles May 26, 1936 2,119,588 Lindenblad June 7, 19382,247,963 Ransburg et a1 July 1, 1941 2,359,476 Gravley Oct. 3, 19442,371,605 Carlton Mar. 20, 1945 2,421,787 Helmuth June 10, 1947

